This paper reports on a multi-phased, mixed-method consensus-based study conducted with young carers in the UK aged 11–18, and health, social care and education professionals from the UK, USA and Canada, to identify priority items for inclusion in a short screening tool for use with young carers of a family member with a progressive or long-term illness or disability. Following ethical approval from University and local Research Ethics Committees, qualitative and quantitative data were collected between 2017 and 2019 from 267 people (107 young carers; 160 professionals), through interviews, a focus group, a Delphi survey, consensus group meetings and consultations. Qualitative data were analysed thematically, and quantitative data were analysed using measures of central tendency, frequency and levels of dispersion. The resulting Carers’ Alert Thermometer (CAT-YC) contains an identification question followed by ten areas of need across two themes of ‘current caring situation’ and ‘carer’s health and wellbeing,’ along with guidance for possible next steps and space for an action plan to be jointly agreed between the screener and young carer. Preliminary piloting of the CAT-YC provides evidence of identifying and monitoring needs, and is expected to be useful for young carers, a wide range of professionals, and organisations that support young carers.